Long distance optical transmission systems generally require that the optical signals flowing in those transmission systems be amplified at periodic intervals along the length of the transmission system. The optical signals may be amplified by a number of repeaters located along the length of the transmission system. Repeatered optical transmission systems generally fall into two broad categories, those using electro-optical regenerators and those using optical fiber amplification. Electro-optical regenerators are not very desirable because they contain a complex array of parts, including a receiver which converts weak optical pulses into electrical pulses which are then amplified, reshaped, retimed, and converted back into light pulses for continued transmission through the system. These complexities can be avoided because recent advances in fiber and optical amplifier technology make possible the linear amplification of light pulses without such electro-optical conversion. Because of their relative simplicity and excellent gain characteristics, optical fiber amplifiers are highly desirable, especially in repeatered undersea systems.
It is crucial that the gain of the fiber amplifiers be carefully controlled for low bit error rates in the transmission system and long service life of the components used in the repeaters. The gain in a fiber amplifier results from interactions, in a specially doped length of fiber, between signal photons having a wavelength .lambda..sub.s with photons produced by a pump laser having a wavelength .lambda..sub.p, where .lambda..sub.s is greater than .lambda..sub.p. The dopant in the fiber, typically erbium, absorbs the power from the pump laser at wavelength .lambda..sub.p and emits optical power under such stimulation at a wavelength .lambda..sub.s. One of the parameters which determines the gain of a fiber amplifier and the output power is the output power of the pump laser which stimulates the doped fiber. Specifically, both the gain and output power increase with increasing pump laser power. The gain and output power should be high enough so that the signal level is significantly above the noise floor. However, the output power cannot be too high or the effect of fiber non-linearities being to intrude.
The performance of a fiber amplifier based transmission system is limited by the presence of amplified spontaneous emission (ASE) noise generated by the amplifier and by the effect of chromatic dispersion and nonlinearities in the transmission fibers. At low signal levels, inadequate signal-to-noise ratio is a concern. At high signal levels, non-linearities are a similar concern. The dominant nonlinear effect is the Kerr effect which causes the refractive index of the fiber to change with light power density through the fiber. This nonlinearity spreads the signal spectrum and mixes it with the ASE noise spectrum. The chromatic dispersion phenomenon then causes communication pulses to spread in time resulting in intersymbol interference. Under both low signal conditions and high signal conditions, therefore, serious signal degradation may result. In any transmission system containing fiber amplifiers, there is a particular level of pump laser power which produces minimum signal degradation and, hence, the lowest bit error rate by insuring optimum signal-to-noise ratio and minimizing the effects of nonlinearities. At lower pump laser power levels, the communications signal level decreases and the level of the ASE noise may make it difficult to differentiate the signals from the noise. At higher power levels, the nonlinear mixing of communication signals and ASE noise becomes important. Therefore, there is an urgent need for economically and effectively controlling the optical output power of each of the pump lasers in an optical fiber amplifier based transmission system.